Mortimus eventually became aware of other worlds where everything he believed in was meaningless, so he turned to politics, attempting to "create a purpose out of nothing". Finding politics to be full of betrayal, he retreated into hedonism, out of a desire for harmless fun. (PROSE: No Future)

The Monk delighted in meddling with history, specifically the history of Earth, to change it for his own amusement and for what he thought the better. He had previously lent mechanical assistance to the builders of Stonehenge by providing anti-gravity lifts; in his logbook it could be read that he gave Leonardo da Vinci tips on aircraft design, and he placed £200 in a bank on Earth in 1968 and then travelled forward two hundred years to pick up a fortune in compound interest. At the time that the First Doctor encountered him, the Monk was trying to prevent the Norman Conquest of 1066 as part of a plan to guide England into an age of technological prosperity. He wore a monk's habit to gain the trust of the locals, hence the name by which he was usually referred.

After thwarting the Monk's plans, the Doctor sabotaged the dimensional circuit of his TARDIS, making it the same size inside as outside. With his TARDIS interior reduced to dollhouse proportions, the Monk seemed stranded in one time and place. (TV: The Time Meddler)

The Doctor ran into him again on the volcanicplanetTigus. The Monk sabotaged the lock on the Doctor's TARDIS, though that did not stop him from getting inside. The Doctor stole the Monk's direction controls to use in his effort to stop the Daleks.

The Monk's TARDIS landed in ancientEgypt. Knowing of the Daleks, the Monk decided to help them regain the taranium core to avoid being exterminated himself, while trying unsuccessfully to convince the Doctor and his companions of his honourable nature. The Doctor overpowered the Monk and placed him in a sarcophagus, where he was found by Steven Taylor and Sara Kingdom. He caused them to be captured by the Daleks, but was also held by them. The Doctor tinkered with the chameleon circuit of the Monk's TARDIS, making it take various shapes, finally that of a police box. The Doctor was forced to give the taranium to Mavic Chen, enabling his companions and the Monk to escape the Daleks. The Monk entered his TARDIS before the Daleks could recapture him. However the Doctor had stolen its partially compatible directional unit, ultimately leaving the Monk stranded on an ice planet. (TV: The Daleks' Master Plan)

Besieged by angry brothers and an irate Doctor, the Monk slipped back into his TARDIS and took off. The Brothers hijacked the Doctor's TARDIS at gunpoint and ordered him to follow the Monk's TARDIS through time. A chase ensued, and the two TARDISes flitted to several famous mysteries in Earth's history. Finally, the Doctor and the Sleeze Brothers made the Monk's TARDIS implode. This apparently caused the creation of the Bermuda Triangle. (COMIC: Follow That TARDIS!)

Mortimus created a series of alternate timelines (PROSE: Blood Heat, The Dimension Riders, The Left-Handed Hummingbird, Conundrum) in a scheme employing the ChronovoreArtemis. Mortimus now used his real name and posed as a 1976 record executive in England. He had also, by this time, made himself servant to a being much more powerful and intelligent than himself, as the Champion of the EternalDeath. Mortimus aided the Vardans' scheme to avenge themselves on the Doctor and the Sontarans by conquering Earth, a planet of continued strategic value to the Sontarans and of importance to the Doctor. His plan was undone thanks to the Doctor's companionAce, who pretended to side with him until she could free Artemis. The vengeful Artemis subsequently took Mortimus away to make him pay for her imprisonment. (PROSE: No Future)

After Kells, Lucie and the Monk went to the planet Questus, where the Monk had travelled back in time kill the parents of a dictator in an avalanche to prevent the dictator's birth. At this point, after being dragged out of the avalanche, she tired of the Monk's meddling, telling him, "I don't wanna hang around with a murderer, 'cause even if wanna try and avoid wars and stuff, I mean, it's not worth it if you have to kill people." After this, the Monk "dumped" her on Deimos Moonbase in the 23rd century, calling Lucie an "ungrateful, interfering, aggravating pest". He had deliberately placed Lucie there in an attempt to grab the Doctor's attention.

The Monk had awoken the Ice Warriors centuries before they were meant to — according to the Eighth Doctor — to make them terraform Mars to how it was, killing thousands of human colonists in the 23rd century, rather than billions on Halcyon a thousand years later. Realising the Doctor hadn't taken his bait, the Monk created an artificial gravity eddy to forcibly bring the Doctor back to Deimos. The Monk took the Doctor's companion, Tamsin Drew, to the aftermath of the Ice Warriors' attack on Halcyon around the 33rd century, convincing her that the Doctor was responsible for the billions of deaths there, and subsequently showing him apparently collaborating with the Ice Warriors in killing 600 people on a passenger rocket.

When the Doctor had put things as they were before, Tamsin told him she had "had enough" of what she considered only looking out for his friends and the Web of Time, and condemning the fate of Halcyon in the future to the Ice Warriors and left with the Monk. The duo set off to find "some old friends who also [had] a score to settle with the Doctor" to "combine their talents". (AUDIO: The Resurrection of Mars)

The Monk helped the Daleks invade Earth as they had in the 22nd century. In exchange for reviving the Dalek Time Controller, the Daleks promised him anything he desired. He used a Dalek virus from the far future and gave Tamsin the job of stealing human artefacts from museums for his personal collection. (AUDIO: Lucie Miller) After the Daleks destroyed the collection in an attempt to kill the Doctor, and then coldly exterminated Tamsin, the Monk was devastated that he had allowed someone he had feelings for to die. Wishing not to cause any more death, he decided to help the Doctor. He saved Susan Campbell and him from the bomb that destroyed the Daleks. He was then ordered to leave the Doctor's sight after the Doctor found out that he had deployed the virus on Earth. (AUDIO: To the Death)

The Monk has the distinction of being the first member of the Doctor's race, besides the Doctor himself and Susan, to appear in the series. The name Time Lord would not be used until The War Games.

For some time there was speculation that the Monk was actually an earlier incarnation of the Master or the War Chief, who appeared in The War Games. This is stated as fact in The Doctor Who Role Playing Game. However, this theory has been contradicted by the Monk's appearances in novels and comics.

This character was almost never actually called "the Time Meddler" or "the Meddling Monk". Both of these "names" are taken from viewer's out-of-universe knowledge of the titles of the overall serial and the second episode. The Doctor comes close to giving the character these names, however, calling him "a time meddler" and "that meddling monk". In the novelisation of Shada he actually refers to "the Meddling Monk", but in general it's more epithet than name. "The Monk", as a name, derives more from Steven and Vicki's attempt to call him something, at least within the confines of The Time Meddler. By the events of the audio story The Book of Kells, it's clear that the Monk has appropriated the name. By the incarnation that faced the Eighth Doctor, he was doing a Master-like wordplay that indicated he thought of himself as "the Monk". He called himself "Abbot Thelonius" — a sly reference to jazz great Thelonius Monk.